CHAP. XII 
A RE NIG ER UP TIONS—MERIONE THS HIRE 
trachytes, while in at least one instance, somewhat low down in the section, 
quartz-grains with intruded material point to the existence of some fairly 
acid and \dtreous lava.^ On the south side of Llyn Cau, that is towards the 
top of the volcanic group, I found a coarse agglomerate with blocks of felsitic 
lavas, sometimes three feet across (see Mg. 48). This gradual increase of 
acidity in the lapilli of the tuff's finds an interesting confirmation in the 
contemporaneous lava-sheets to which I shall afterwards allude. 
One of the most noticeable features in the tuffs of this volcanic group 
is the great abundance of entire and broken crystals dispersed through them. 
These crystals have certainly not been formed m situ, hut were discharged 
from the vents as part of the volcanic dust. They usually consist of felspar 
which, at least in the southern portion of the district, appears generally to 
be plagioclase. Frequent reference to these crystals as evidence of volcanic 
explosions may be found in the publications of the Survey. Nowhere can 
they be better seen than in the black slate-tuff's of Cader Idris. They are 
there white, more or less kaolinized, and as they lie dispersed through the 
black base, they give the rock a deceptive resemblance to some dark 
porphyry. The large crystals of hornblende and augite abundantly scattered 
through much of the tuff of Ehobell Fawr have been already referred to. 
In the central parts of the district thick bands of asbes were mapped 
by the Survey, and described as consisting almost wholly of volcanic materials, 
but containing occasional thm bands of slate which suffice to mark pauses 
in the eruptions, when ordinary sediment was strewn over tlie sea-bottom. 
In the Cader Idris gi’ound, on the other hand, interstratifications of non- 
volcanic material are of such frequent recurrence as to show that there, 
instead of constant and vigorous discharges accumulating a vast pile of 
ashes, the eruptions followed each other after intervals of sufficient duration 
to allow of the usual dark sediment spreading for a depth of many feet 
over the sea-bottom. 
One of the most interesting deposits of these interludes of quiescence is 
that of the pisolitic ironstone and its accompanying strata on' the north 
front of Cader Idris {i in Fig. 48). A coarse pumiceous conglomerate with 
lai’ge slag-like blocks of andesite and other rocks, seen near Llyn-y-Gadr, 
passes upward into a fine bluish giit and shale, among wliich lies tlie bed 
of pisolitic (or rather oolitic) ironstone which is so widely diffused over 
North Wales. The finely-oolitic structure of this band is obviously original, 
but the substance was probably deposited as carbonate of lime under quiet 
conditions of precipitation. The presence of numerous small lAngvla; in 
the rock shows that molluscan life flourished on the spot at the time. The 
iron exists in the ore mainly as magnetite, the original calcite or aragonite 
having been first replaced by carbonate of iron, which was subsequently 
broken up so as to leave a residue of minute cubes of magnetite.^ 
Above the ironstone some more blue and black shale and grit pass under 
* Op. dt. ji. 429. A tiift' lying below the ironstone near Cross Foxes, east of Dolgelly, likewise 
contains fragments of traeliytic lavas. 
- Messrs. Cole and Jennings, op. cit. p. 426. 
